[M-TT48] 地球化学の最前線

[MTT48-02] SIMSによる玄武岩質ガラスの水素、硫黄同位体比分析法とその応用

*清水 健二¹、牛久保 孝行¹、村居 呂一²、松浦 史宏²、上野 雄一郎² (1.海洋研究開発機構　高知コア研究所、2.東京工業大学)

キーワード：SIMS、水素同位体比、硫黄同位体比、玄武岩質ガラス

We developed rapid and accurate in situ analyses of hydrogen and sulfur isotope ratios of basaltic glass using high-resolution, multi-collection secondary ion mass spectrometry (CAMECA IMS-1280HR). Hydrogen and sulfur isotopes of standard basaltic glasses were determined by a high-temperature conversion elemental analyzer/isotope ratio mass spectrometer (IRMS) and IRMS, respectively. For the in situ analysis of sulfur isotopes, a defocused Cs beam (~0.5 nA; ~10 µm diameter) was used, but for hydrogen isotopes, we used a larger defocused beam (~5 nA; ~15 µm diameter) to decrease the hydrogen background. For analyses of D/H (³⁴S/³²S) ratios, ¹⁶OH (³²S) and ¹⁶OD (³⁴S) were measured in multi-detection mode with a Faraday cup and an axial electron multiplier, respectively. Each measurement lasted 6–7 minutes. Precisions (2 standard errors) for D/H and ³⁴S/³²S ratios were ~6 ‰ (H₂O > 1 wt%) and ~0.6 ‰ (S > 1000 ppm), respectively. Our developed method for rapid and high spatial resolution analysis can determine concentrations of volatiles, hydrogen and sulfur isotopes in a single small melt inclusion of ~30 µm diameter. Using this method, we analyzed hydrogen and sulfur isotope ratios of submarine basaltic glasses from mid-oceanic ridges and oceanic islands of Hawaii and confirmed that their D/H and S isotope ratios were consistent with reported values.